Most modern electronic flash units use a triggering electrode method of flash firing. In this type of circuit, a main capacitor is connected permanently across two terminals of a glass flash tube filled with inert xenon gas. The resistance of the gas in the flash tube is normally too high to permit a direct discharge. For firing the flash unit there is a third electrode--for example, a coil of wire wound around the outside of the flash tube. An instantaneous triggering voltage of about 5000 volts applied to the coil of wire ionizes the gas in the tube, thus lowering the resistance of the gas and allowing the main capacitor to discharge its energy through the tube in the form of a intense flash of light. The triggering voltage is usually supplied by a trigger or ignition coil connected to a small capacitor.
Instead of winding the coil of wire around the outside of the flash tube, the tube can be supported with its outside directly against the inner wall of an electrically conductive flash reflector that is connected to the trigger coil as mentioned in U.S. Pat. No. 3,484,597, issued Dec. 16, 1969, and U.S. Pat. No. 4,570,203, issued Feb. 11, 1986. In this instance, the flash reflector serves as a light reflector and as a triggering electrode. Typically, a piece of wire or metal tape has one end soldered to a circuit board, which includes the trigger coil, and another end secured to the flash reflector. Ways are continuously being devised to simplify manufacture and design of the flash unit.